The invention relates to a process for the preparation of gas oil from an asphaltenes-containing hydrocarbon oil by thermal cracking.
In the atmospheric distillation of crude mineral oil, as practiced on a large scale in refineries for the preparation of gasolines, kerosines and gas oils, an asphaltenes-containing oil is obtained as a by-product. In view of the increasing need for the above-mentioned hydrocarbon oil distillates and the decreasing reserves of crude mineral oil, several processes have been proposed aiming at the conversion of the asphaltenes-containing oils, which were at first used substantially as fuel oil, into hydrocarbon oil distillates. Examples of such processes are catalytic cracking, thermal cracking, gasification in combination with hydrocarbon synthesis, coking and hydrocracking. Previously, Applicant's employer developed an attractive process for the preparation of gas oil from asphaltenes-containing hydrocarbon oils by thermal cracking. This process is carried out in an apparatus system which comprises the first thermal cracking unit, a cyclone phase separation unit, an atmospheric distillation unit (in which, if desired, the distillation can be carried out at a maximum pressure of 5 bar) and the second thermal cracking unit. In the process the asphaltenes-containing hydrocarbon oil is converted by thermal cracking in the first thermal cracking unit into a cracked product which comprises 5-30%w of components boiling below the boiling range of the feed. The cracked product is separated in the cyclone unit into a light fraction boiling substantially below 500.degree. C. and which contains, in addition to components boiling below 350.degree. C., both light and heavy components boiling between 350.degree. and 500.degree. C., and into a heavy fraction boiling substantially above 350.degree. C. and which contains, in addition to components boiling above 500.degree. C., both light and heavy components boiling between 350.degree. and 500.degree. C. The light fraction from the cyclone unit is mixed with the cracking product from the second thermal cracking unit and the mixture is separated in the atmospheric distillation unit into a number of light distillate fractions of which the heaviest is the desired gas oil, a heavy distillate fraction and a residual fraction. The heavy distillate fraction from the atmospheric distillation unit is converted in the second thermal cracking unit into a cracking product which consists of 20-75%w of components boiling below the boiling range of the feed for the first thermal cracking unit.
Although the above-described process offers the possibility of preparing a high-grade gas oil from an asphaltenes-containing hydrocarbon oil as the starting material, it has the drawback that the yield of atmospheric distillates is low. Now Applicants have carried out an investigation to find measures by which the yield of atmospheric distillates can be increased using the above-described process. In this investigation it was found that the heavy fraction which is separated in the cyclone unit consists to a considerable extent of components which are very suitable for use as the feed for the second thermal cracking unit. These components may partly be isolated from the heavy fraction by subjecting said heavy fraction to vacuum distillation and then by subjecting the vacuum residue obtained in this vacuum distillation to a deasphalting step. The mixture of vacuum distillate and deasphalted oil was found very suitable for use as feed component for the second thermal cracking unit.